Aluminum killed-steel

ABSTRACT

AN AL-KILLED STEEL CONSISTING OF A SI-AI-NB SYSTEM THAT CAN BE PRODUCED WITH A GOOD YIELD AND HAS EXCELLENT SURFACE QUALITY FOR PROVIDING A LINE PIPE TENSILE STRENGTH OF MORE THAN 60 KG./MM.2, GOOD LOW TEMPERATURE TOUGHNESS AND DISTINQUISHED WELDABILITY.

United States Patent O 3,681,057 ALUMINUM KILLED-STEEL Kiminari Kawakam and Shizunao Yamagishi, Fukuyama, Japan, assignors to Nippon Kokan Kabushiki Kaisha, Tokyo, Japan .Filed June 10, 1970, Ser. No. 45,108 Claims priority, application Japan, June 12, 1969, y44/ 45,748 Int. Cl. C22c 39/20 U.S. Cl. 75-124 1 Claim ABSTRACT OF THE DISCLOSURE An -Al-killed steel consisting of a Si-Ai-Nb system that can be produced with a good yield and has excellent surface quality for providing a line pipe tensile strength of more than 60 kg./mm.2, good low temperature toughness and distinguished weldability.

The present invention is concerned with an aluminum killed-steel of excellent quality and more particularly to Al-killed steel of an improved composition.

lIn recent years the demands for line pipe that is used for the extended transportation of crude oil petroleum products, natural gas and the others has more and more increased, and the diameter of said pipe has increased. It is well known that a welded pipe, Which is made of plate, is widely employed as said line pipe. The size and quality of these materials are specified in known API Standards, e.g. A-P'I-5LX-60, API-LX-65, etc. In such steels, it is needless to say that the workability and weldability as well as the mechanical properties of said line pipe are required to be of high quality. Special attention should be paid to the low-temperature notch toughness of said steels, when services in the coldest regions are to be considered. It will be understood that the chemical composition of said API Standards is presently insufiicient since these Standards may be tentative and leave room for development. It is well known that the composition of API x-series Standards shall conform to the chemical requirements agreed upon between the purchaser and manufacturer.

This invention has mechanical properties as specified is said API Standards. The features lie in the addition of low silicon, Sol. A-l and higher content of Nb, without addition of costly V, Ti and the like.

An object of this invention is to provide an Al-killed steel having a high yield point of more than 65,000 p.s.i. that can be produced with ease and stability.

Another object of this invention is to provide an Alkilled steel having excellent toughness in low temperature services.

'Other objects and advantages of the invention will be apparent from the following description and the accompanying drawing, in which FIG. 1 is a histograph showing the surface quality of ordinary steel;

FIG. 2 is a histograph from cold ingot treatment; and

'FIG. 3 is a histograph of the present invention ingot.

Steel of the present invention is produced as an Alkilled-steel. General, surface cracks of a slab, which is shaped from said Al-killed ingot, are observed. Such cracks are generated by the precipitation of AIN, which is formed -by the cooling of the surface layer of the hot ingot. It is needless to say that said precipitation goes on as the track time becomes longer. According to many experments, it has been discovered that said generation of cracks can be avoided by means of adding low Si, instead of cold ingot treatment that is cooled to room temperature and then charged into a soaking pit. That is, a

small content of Si is effective to prevent the ingot from precipitation of AIN at the cooling stage of said ingot. The Si content of more than 0.15 has little effect on preventing the ingot from precipitation of AIN. When the Si content is less than 0,02% the steel increases in inclusion and results in dirty steel. A lowering of yield will be inevitable.

The C content of the present invention is limited to a range from 0.13 to 023%. A C content of more than 0.13% is required in order that the tensile strength of more than 60 l g./mm.2 is kept with stability. However, when the C content is above 023%, weldability of the steel will be lower. Such a steel is undesirable, because weldability of the steel is an important factor of the invention.

An Mn content of more than 1.5 makes weldability of the steel worse in connection with the C content. The tensile strength of the steel including the Mn content of less than 12% becomes insufcient in connection with the |C content. Accordingly, the range of the Mn content of the invention is from 1.2 to 15%.

.Effects of Nb added to steel lie in the precipitation effect in ferrite grain and grain refining effect by Nb carbide. The best suited range of Nb content in the invention steel is from 0.01 to 0.05 Even though more than 0.05% Nb was added, larger effects cannot be expected, :because said effects amount to a saturated condition, and less than 001% Nb has no effect.

A Sol. Al content in the range from 0.02 to 0.05% has the highest grain refining effect. Even though said content was above 0.05 there is no said effect being in proportion to said content. When Sol. Al content is less than 0.02%, said grain refining effect cannot be expected.

Mechanical properties in a low temperature services, eg. low temperature notch toughness, are possible with the above mentioned grain refining effect of Nbcar-bide and Sol. Al content.

Number of steel: 4

Analysis of crude steel: as mentioned in Table 4 Steel making furnace: basic oxygen furnace Ingot weight, slab size and plate dimension: as mentioned in Table 1 Mechanical properties: as mentioned in Table ll Merits of the invention steel, which is confirmed in the above experments, are as follows:

(1) Cold ingot treatment does not need to be used. Accordingly, an increase in the capacity of soaking pit is 6 t./hr.

(2) There is little surface crack in the slab.

(3) Cleanliness of ingot and low temperature toughness of steel are remarkably improved.

(4) Resistibility for hydrogen brittleness increases and white spot of slab and plate decreases.

(5) Defects that are apt to be generated at welded portions can be checked.

Each of the accompanying drawing figures is a histograph of the surface quality of slabs. Axis of abscissas in said figures shows the rate of reduction of weight, which is based on the repairing operation, with a rank. Said reduction of weight shown as numeral l is less than 05%, numeral 2 is from 0.5 to 0.1%, numeral 3 is from 1.0 to 20%, numeral 4 is from 2.0 to 2.5 numeral 5 is more than 2.5 and numeral 0 is impossible to be repaired, which is scrapped.

According to the above histograph it will be understood that the surface quality of the invention is excellent in comparison with other prior steels.

This invention steel is possible to be widely utilized as structural materials, in addition to use as materials for line pipe.

ICC 

